In what was is the prison presented as a metonymy in the poem, To Althea, From Prison?

The prison itself is a metonymy, which is a type of symbol that represents itself while also standing for a larger idea. In this case, the prison represents the constraints others place on us. These constraints may be quite literal (like a prison), or they may be metaphorical, like societal expectations. Either way, the speaker suggests, freedom lies in overcoming them.